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INTERNET-DRAFT                           Erik Nordmark, Sun Microsystems
Februari 7, 2001


                    Site prefixes in Neighbor Discovery

                 <draft-ietf-ipngwg-site-prefixes-05.txt>


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  sp Internet-Drafts are
   working documents of the Internet Engineering Task Force (IETF), its
   areas, and its working groups.  Note that other groups may also
   distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet Draft expires August 7, 2001.



Abstract

   This document specifies extensions to IPv6 Neighbor Discovery to
   carry site prefixes.  The site prefixes are used to reduce the effect
   of site renumbering by ensuring that the communication inside a site
   uses site-local addresses.

   This protocol requires that all IPv6 implementations, even those that
   do not implement this protocol, ignore all site-local addresses that
   they retrieve from the DNS when the AAAA or A6 RRset contain both
   global and site-local addresses.  If the RRset contains only site-
   local addresses those addresses can be used.







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Contents

   Status of this Memo..........................................    1

   1.  INTRODUCTION AND MOTIVATION..............................    3

   2.  TERMINOLOGY..............................................    4
      2.1.  What is a Site?.....................................    4
      2.2.  Requirements........................................    5

   3.  OVERVIEW.................................................    5
      3.1.  Protocol Overview...................................    5
      3.2.  Mobile IP Implications..............................    7
      3.3.  Assumptions.........................................    9

   4.  UPDATED PREFIX OPTION FORMAT.............................   10

   5.  CONCEPTUAL VARIABLES.....................................   11

   6.  SENDING RULES............................................   12

   7.  RECEIVING RULES..........................................   12

   8.  USING THE SITE PREFIXES..................................   13
      8.1.  Host Name Lookups...................................   13
      8.2.  IPv6 Address Lookups................................   15

   9.  MULTI-SITED NODES........................................   16
      9.1.  Detecting that a Node is Multi-sited................   16
      9.2.  Address Records for Multi-sited Nodes...............   17
      9.3.  Distinguishing Between Different Sites..............   18

   10.  SECURITY CONSIDERATIONS.................................   18

   REFERENCES...................................................   19

   AUTHOR'S ADDRESS.............................................   20

   APPENDIX A: CHANGES SINCE PREVIOUS DRAFT.....................   21












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1.  INTRODUCTION AND MOTIVATION

   In order to maintain the aggregation of the global Internet routing
   tables it might be necessary for whole sites to renumber to use
   different prefixes for their global IPv6 addresses.  Such renumbering
   would not directly benefit the renumbered sites but instead be
   necessary for the scaling of the Internet as a whole.

   In order to increase the probability that such renumbering is viewed
   favorably by the sites themselves, which see little or no direct
   benefit, it is critical that both the effort of renumbering is kept
   at a minimum and also that the risk associated with renumbering is as
   small as possible.

   The Stateless address autoconfiguration [ADDRCONF] and support for
   router renumbering [ROUTER-RENUM] make it easier to renumber a site.
   However, these protocols do not by themselves address long-running
   TCP connections or cases where IP addresses have been stored in some
   configuration file.  Thus additional measures are needed to reduce
   the cost of renumbering.

   For many sites it is much more critical to maintain the internal
   communication than the inter-site communication over the Internet.
   Based on that observation this proposal tries to limit the effect of
   a site renumbering one or more of its global prefixes by ensuring
   that intra-site communication can use site-local addresses which
   would not be affected by the site renumbering.  With this proposal it
   is possible to maintain internal long-running TCP connections or
   otherwise store IPv6 addresses for longer time than would have been
   possible without it.

   As specified in [ADDR-TODAY] IP addresses are no longer temporally
   unique.  This implies, among other things, that applications should
   not store IPv6 addresses without a mechanism for honoring the DNS
   time-to-live and refreshing the IPv6 address.  This protocol is not
   intended to deter from that recommendation but is merely based on the
   observation that the applications today might assume that IPv4
   addresses are temporally unique and it is likely that some
   applications might not be corrected in their behavior as they are
   moved to IPv6.  It would be unfortunate if such application
   "brokenness" would lead sites to view site renumbering as a too risky
   or a too costly operation.

   This document does not address the general issues of renumbering such
   as renumbering a single host or a subnet.  It is targeted at site
   renumbering.  The proposal does not attempt to address how long-
   running TCP connections going outside a site will survive the site
   renumbering.



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   Note that using literal site-local addresses would survive a site
   renumbering event but it would not survive other forms of renumbering
   e.g. subnet or host renumbering.  Thus nodes should use host names
   instead of literal addresses for the same reasons as specified in
   [ADDR-TODAY].

   The author would like to acknowledge the contributions the IPNGWG
   working group and in particular Mike O'Dell who pointed out the
   importance of the problem, and Robert Elz who suggested this approach
   to solving the problem.


2.  TERMINOLOGY

   This documents uses the terminology defined in [IPv6] and [DISCOVERY]
   and in addition:

      Multi-sited node
                    A node that has interfaces in multiple sites.


2.1.  What is a Site?

   This document does not attempt to define the concept of a "site", but
   it does place some assumptions on such a definition.  These
   assumptions are consistent with [SCOPE-ARCH]:

    - A site is an administratively controlled piece of topology that is
      well-connected.  It can be connected using tunnels including the
      special form of tunnels (using routing headers and home address
      options) defined in [MOBILE-IPv6].

    - A link can belong to zero or one site.  This implies that an
      interface can belong to at most one site.

    - A node can have interfaces belonging to different sites.  Such a
      node is said to be multi-sited.

    - A mobile node [MOBILE-IPv6] which has been assigned one or more
      site-local addresses and moves outside the site which contains its
      home address (its "home site") is considered to have one
      interfaces which is part of the "home site".









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2.2.  Requirements

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [KEYWORDS].

3.  OVERVIEW

   The goal of this extension to Neighbor Discovery is to make
   communication that is local to a single site use the site-local
   addresses instead of the global addresses.  If all communication
   internal to a site uses site-local addresses then the site's global
   addresses can be renumbered without having any affect on the internal
   communication.  Thus the risk associated with site renumbering is
   lowered - applications that store IPv6 addresses and long-running TCP
   connections will, as long as the communication is local to the site,
   continue to operate across the renumbering of the site.

   A few alternative solutions have been explored.  An early proposal
   was to place the site-local addresses in the name service (e.g., the
   DNS) and make sure they are returned first in the list of addresses
   returned to an application (to make it likely that the application
   will use that address).  That proposal has the disadvantage that the
   name service must return different addresses depending on who asks
   the question; if a node inside the site asks for an address it should
   return the site-local address(es) but if a node outside the site asks
   it must not return a site-local address.  This is referred to as the
   two-faced DNS.  While some sites use a two-faced DNS today as part of
   their firewall solution it would be rather unfortunate if each and
   every site had to deploy such a solution.  See [GSE-EVAL] for more
   discussion.

   An earlier version of this proposal took a different approach.  The
   name service would only contain global addresses and the routers
   would advertise the global address prefixes assigned to the site
   which the nodes would use to derive site-local addresses
   corresponding to the global addresses returned from the name service.
   That approach had the disadvantage that all nodes in a site would be
   required to respond to their automatically derived site local
   address.  For instance, it was not possible to have certain mobile
   nodes that would only be reachable using global addresses.


3.1.  Protocol Overview

   This version of the document takes a middle ground.  The site-local
   addresses, as well as the global addresses, are stored in the DNS
   without requiring a "two-faced" DNS.  All nodes are required to



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   ignore any site-local addresses retrieved from the DNS unless:

    1) the DNS returned only site-local addresses (used in sites that
       are not connected to the Internet i.e. where all the addresses
       are site-local), or

    2) they can determine that they are in the same site as the peer.
       This determination is done by verifying that the retrieved
       AAAA/A6 RRset for the peer includes one or more global addresses
       that match the site prefixes advertised by the routers.

   This protocol assumes that the routing infrastructure will be used to
   distribute information about which prefixes belong to the local site.
   This document only specifies how the site prefixes are distributed
   from the routers to the hosts on each link.  However, other protocols
   such as [ROUTER-RENUM] might be extended to carry the site prefixes
   to all routers in a site.  The use of the routing infrastructure to
   carry the site prefixes avoids the "two-faced" issue above - the
   routers know which part of the network is inside the site thus they
   can naturally prevent this information from being distributed outside
   the site.

   The protocol is based on each host maintaining a list of all the
   currently active site prefixes.  The site prefixes are periodically
   advertised in Neighbor Discovery Router Advertisement messages and
   each prefix has an associated lifetime.

   Once a host has a list of prefixes that apply to its site it uses
   this information to determine if the global addresses contained in a
   AAAA/A6 RRset is part of its site.  If this is the case then the host
   can use any site-local addresses contained in that AAAA/A6 RRset.
   Otherwise any site-local addresses contained in that RRset must be
   ignored.  A node should prefer the site-local addresses over the
   global addresses e.g. by having the applications try the site-local
   addresses before or instead of the global addresses.

   The reverse lookup (from an IPv6 address to a host name) is handled
   by mapping a site-local address to the corresponding global addresses
   as a fallback.  Thus, if the address being looked up is a site-local
   address and the reverse lookup for that address fails the host
   constructs the corresponding global addresses using the list of site
   prefixes and performs a reverse lookup on those addresses until a
   match is found.

   It is expected that both the forward and reverse lookup rules can be
   hidden from applications by implementing them as part of the library
   that handles host name lookups.




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3.2.  Mobile IP Implications

   A mobile node which moves outside its "home site" must maintain the
   "home site-local addresses" for continued communication with nodes in
   its "home site".  This implies that such a mobile node conceptually
   will have one interface (for the traffic destined to and from its
   home site) which is assigned the home site-local addresses in
   addition to its other interfaces which might be part of the visited
   site.

   A mobile node may choose to autoconfigure site-local addresses in the
   visited site.  However, such addresses add complexity to the mobile
   node with little or no benefit.  Thus it is recommended that mobile
   nodes only autoconfigure global addresses when moving to links
   outside its home site.

   A mobile node needs to be able to detect when it has moved to a
   different site.  Thus in addition to the regular movement detection
   in [MOBILE-IPv6] it should inspect the site prefixes in the Router
   Advertisement messages to determine when it is outside its home site.

   The remainder of this section specifies the operation of Mobile IP
   when the mobile node is outside its home site.

   The mobile node needs to retain any site-local addresses it was
   assigned in its home site, but those site-local addresses should only
   be used when communicating with nodes in its home site.

   The binding updates must use a global address as the care-of-address.

   There are no changes needed to Home Agents.  The home agent needs to
   select a proper source address when sending to a global address as is
   expected of all IPv6 implementations - it should not use a site-local
   source address when sending to a global destination address.

   The only change needed to the Correspondent Nodes is to not use a
   site-local source address when sending to a global destination:  When
   using a Routing Header to communicate with a mobile node that has a
   global Care-of-Address the correspondent needs to include a Home
   Address Option to carry its site-local source address and set the IP
   source address field to one of its global addresses.

   This additional use of the Home Address Option from the
   correspondents ensures that all traffic to and from the mobile node
   will have global source addresses.  Thus the site-local addresses
   will be "hidden" in 1) encapsulated headers, 2) routing headers, or
   3) home address option.




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   Packets encapsulated to the mobile node will look like this:

      Outer IP header destination address:
                Registered care-of-address.  A global address.

      Outer IP header source address:
                Global address assigned to home agent

      Inner IP header destination address:
                One of the mobile node's home addresses.  Likely to be a
                site-local address.

      Inner IP header source address:
                Sender of original packet.  Likely to be a site-local
                address.


   Packets sent to the mobile node using routing headers:

      IP header destination address:
                Registered care-of-address.  A global address.

      IP header source address:
                A global address is needed to match the scope of the
                destination address.  (This requirement is added by this
                specification.)

      Routing header:
                The mobile node's home address which has been used for
                this communication e.g. which identifies the TCP
                connection.  Likely to be a site-local address.

      Home address option (This requirement is added by the
                specification.):
                The correspondent node's address which has been used for
                this communication e.g. which identifies the TCP
                connection.  Likely to be a site-local address.


   Packets sent from the mobile node to a site-local correspondent
   address:

      IP header destination address:
                The correspondent node's global address.  If this is not
                known then the packet must be instead be encapsulated
                and sent to the (global address of) the home agent which
                can deliver the packet to the site-local destination
                address.



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      IP header source address:
                Mobile node's care-of-address.  A global address.

      Routing header:
                The correspondent node's address which has been used for
                this communication e.g. which identifies the TCP
                connection.  Likely to be a site-local address.

      Home address option:
                The mobile node's address which has been used for this
                communication e.g. which identifies the TCP connection.
                Likely to be a site-local address.


   Packets sent from the mobile node to a global correspondent:

      IP header destination address:
                The correspondent node's global address.

      IP header source address:
                Mobile node's care-of-address.  A global address.

      Home address option:
                The mobile node's address which has been used for this
                communication e.g. which identifies the TCP connection.
                Likely to be a site-local address.



3.3.  Assumptions

   The protocol assumes that the site uses a consistent subnet numbering
   scheme across all its global addresses and its site-local addresses.

   Thus, for every subnet in the site that uses both global and site-
   local addresses, the 16-bit subnet ID field [ADDR-ARCH] for the
   site-local address must have the same value as the Site-Local
   Aggregator(s) field in the global addresses.  However, it is possible
   that some hosts (or whole subnets) only be configured with site-local
   addresses in which case they will only be reachable from nodes within
   the site.  Is it also possible that some hosts (or subnets) only be
   configured with global addresses in which case they will not benefit
   from use of site locals.








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4.  UPDATED PREFIX OPTION FORMAT

   The protocol adds two new fields using previously reserved parts of
   the Prefix Information Option defined in [DISCOVERY].

         0                   1                   2                   3
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |     Type      |    Length     | Prefix Length |L|A|R|S|Resvd1 |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                         Valid Lifetime                        |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                       Preferred Lifetime                      |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |           Reserved2                           | Site PLength  |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
        |                                                               |
        +                                                               +
        |                                                               |
        +                            Prefix                             +
        |                                                               |
        +                                                               +
        |                                                               |
        +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   New fields:

      S              1-bit site prefix flag.  When set indicates that
                     this prefix, in addition to what might be specified
                     by the L and A flags, should be used as specified
                     in this document (to accept and prefer site-local
                     addresses) when an address matches the first Site
                     PLength bits of the prefix.

      Site PLength   8-bit unsigned integer.  This Site Prefix Length is
                     only valid when the S flag is set.  The number of
                     leading bits in the Prefix that are valid.  The
                     value ranges from 0 to 128.  Note that bits in the
                     Prefix past Site PLength bits can be non-zero and
                     MUST be ignored when comparing against the site
                     prefix.

   The defined format above allows a single Prefix Information option to
   carry a subnet prefix used for on-link and/or stateless address
   autoconfiguration [ADDRCONF] together with a site prefix since the
   site prefix(es) are normally sub-prefixes of the subnet prefixes.

   For example, if the subnet prefix is



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           2000:1:2:653a::0/64
   and the site prefix is:
           2000:1:2::0/48
   this can be encoded in a single Prefix Information option with Prefix
   Length being 64, Site PLength being 48, the Prefix being
   2000:1:2:653a::0, and the S flag being set.



5.  CONCEPTUAL VARIABLES

   This document makes use of internal conceptual variables to describe
   protocol behavior and external variables that an implementation must
   allow system administrators to change.  The specific variable names,
   how their values change, and how their settings influence protocol
   behavior are provided to demonstrate protocol behavior.  An
   implementation is not required to have them in the exact form
   described here, so long as its external behavior is consistent with
   that described in this document.

   Hosts will need to maintain the following pieces of information.
   Like the prefix related information specified in [DISCOVERY] this
   information recorded per interface but, except for multi-sited nodes,
   used as a global list being the union of the information over all
   interfaces.  Multi-sited nodes need to use the information separately
   for each site i.e.  form the union over all interfaces that are
   attached to a particular site.  See Section 9 how multi-sited nodes
   operate.

      Site Prefix List

                     A list of the site prefixes that have been received
                     in Router Advertisement messages that have not yet
                     timed out.  Each entry has an associated
                     invalidation timer value (extracted from the
                     advertisement) used to expire site prefixes when
                     they become invalid.  A special "infinity" timer
                     value specifies that a prefix remains valid
                     forever, unless a new (finite) value is received in
                     a subsequent advertisement.

                     Note that the Site Prefix List is separate from the
                     list of on-link prefixes called Prefix List in
                     [DISCOVERY].

   The conceptual Router variable called AdvPrefixList in [DISCOVERY] is
   extended to also contain site prefixes.  Conceptually this can be
   done by having each prefix both contain a AdvSubnetPrefixLength (the



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   length of the AdvPrefix as specified in [DISCOVERY]) and a
   AdvSitePrefixLength field.  If one of the length fields is zero the
   prefix is not used as a on-link and/or addrconf prefix or a site
   prefix, respectively.  The same lifetime values will apply to both
   the subnet and site prefix aspects of a prefix in the AdvPrefixList.

   The above are conceptual variables; Implementations are free to
   implement the router variables as a separate list for the site
   prefixes and the existing Neighbor Discovery AdvPrefixList for subnet
   prefixes.  However, it is desirable that such implementations still
   use a single Prefix Information option to encode both a site and a
   subnet prefix when the site prefix is just a sub-prefix of the subnet
   prefix (unless the lifetimes need to be different for the subnet and
   site prefixes).


6.  SENDING RULES

   When a router is sending Prefix options as part of sending Router
   Advertisement messages, in addition to the rules in [DISCOVERY], the
   router performs the following operations:

    o If the AdvSitePrefixLength field in the AdvPrefixList entry is
      non-zero set the S flag in the Prefix option to one and set the
      Site PLength to the AdvSitePrefixLength.

    o Only if the AdvSubnetPrefixLength field is non-zero should the L-
      bit and the A-bit be set from the AdvOnLinkFlag and the
      AdvAutonomousFlag fields, respectively.

    o The Prefix field and the lifetime fields are set is specified in
      [DISCOVERY].


7.  RECEIVING RULES

   The host receiving a valid Router Advertisement follows the rules as
   specified in [DISCOVERY] with the following additions when processing
   each received Prefix Information option.  For each prefix that has
   the S-flag set:

    o If the Site PLength is zero then do nothing further.

    o If the prefix is a link-local or a site-local prefix then do
      nothing further.

    o If the prefix is a multicast address then do nothing further.




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    o If the prefix is not already present in the Site Prefix List and
      the Valid Lifetime is zero, then do nothing further.

    o If the prefix is not already present in the Site Prefix List and
      the Valid Lifetime is non-zero, then create a new entry for the
      prefix in the Site Prefix List and initialize its invalidation
      timer to the Valid Lifetime value in the Prefix Information
      option.

    o If the prefix is already present in the host's Site Prefix List as
      the result of a previously-received advertisement, then reset its
      invalidation timer to the Valid Lifetime value in the Prefix
      Information option.  If the new Lifetime value is zero, then
      immediately remove the prefix from the Site Prefix List.

   The bits in the Prefix after the first Site PLength bits MUST be
   ignored when the prefix is entered in the Site Prefix List and/or
   when it is compared against other site prefixes.  These bits might be
   non-zero when the Prefix option carries a subnet prefix in addition
   to a site prefix.

   Timing out a site prefix from the Site Prefix List SHOULD NOT affect
   any existing communication.  New communication will use the updated
   Site Prefix List after performing a host name lookup.


8.  USING THE SITE PREFIXES

   The following rules apply when a node looks up host names and
   addresses in a name service such as DNS.



8.1.  Host Name Lookups

   The node will inspect the AAAA/A6 RRset returned from DNS to check if
   one or more of the global addresses belong to the same site as
   itself.  This is done by comparing all the global addresses against
   all the prefixes in the Site Prefix List.  If there are no matches
   then the site-local addresses in the RRset must not be used.  If
   there are one or more matches then the node should prefer using the
   site-local address(es) over the global addresses.  This can be done
   by sorting the addresses before they are returned to the application
   and excluding the addresses that are subsumed by the site-local
   addresses.

   It is important that the site-local addresses are first in the sorted
   list so that the applications try the site-local addresses before any



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   global address.  Also, the matched global addresses are removed from
   the list in order to prevent the applications from using global
   addresses for communication that is local to the site.  An
   alternative would be to keep both global and site-local addresses in
   the list and order the list so that site-local addresses appear first
   i.e.  will be tried first by the application.  The disadvantage of
   doing that approach is that ot doesn't guarantee site-local
   communication uses site-local addresses.  For instance if a router is
   broken when the application tries to connect the application might
   fail to connect using the site-local address.  When it tries to
   connect using the global address the router might be back up.

   A possible algorithm for doing these comparisons is as follows:

      1) Assume the name service returns the global addresses G1, G2,
         G3, ... Gn and the site-local addresses SL1, SL2, ... SLk.
         Assume the prefixes in the Site Prefix List are SP1, SP2, ...
         SPm.  The Site PLength of each of the prefixes is Length(SPj).

      2) If n is zero (i.e. no global addresses were returned) just hand
         all the site local addresses SL1, .. SLk to the application.

      3) Otherwise; for each Gi compare it against all the SPj.  If the
         first Length(SPj) bits of Gi are equal to the first Length(SPj)
         bits of SPj then we have a match.  If there is a match then
         suppress Gi (do not hand it to the application).

      3a) If there is one or more matches then give the application the
         site-local addresses SL1, SL2, ... SLk inserted before the Gi
         addresses that were not suppressed by rule 2)

      3b) If there is no match the result is that the application only
         gets the global addresses G1, ... Gn.

   For example, if the name service returns these addresses for a
   multihomed node:
           2837:a:b:987:X:Y:W:Z1
           2000:1:2:987:X:Y:W:Z1
           fec0::987:X:Y:W:Z1
           2837:a:b:34:X:Y:W:Z2
           2000:1:2:34:X:Y:W:Z2
           fec0::34:X:Y:W:Z2
           2abc:77:66:23:X:Y:W:Z3

   and the prefixes in the Site Prefix List are:
           2837:a:b::0/48
           2000:1:2::0/48




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   The resulting list that the application should use should be:
           fec0::987:X:Y:W:Z1
           fec0::34:X:Y:W:Z2
           2abc:77:66:23:X:Y:W:Z3

   If there is no match (e.g., the Site Prefix List is empty) the
   resulting list that the application should use should be:
           2837:a:b:987:X:Y:W:Z1
           2000:1:2:987:X:Y:W:Z1
           2837:a:b:34:X:Y:W:Z2
           2000:1:2:34:X:Y:W:Z2
           2abc:77:66:23:X:Y:W:Z3

   Note that a minimal implementation can avoid these rules, and not see
   the benefits of the mechanism.  Such implementations MUST ignore
   site-local addresses in the RRset unless the RRset contains no global
   addresses.  Thus they would only use site-local addresses in the case
   when the site is not connected to the Internet.




8.2.  IPv6 Address Lookups

   It is not sufficient to handle the forward lookup.  For instance, the
   node that receives packets and/or connections from a site-local
   address might have the desire to perform a reverse lookup to get a
   host name.  Thus these rules allow such a reverse lookup to succeed
   as long as the Site Prefix List contains all the prefixes that apply
   to the site.

   A possible algorithm for doing this is as follows:

      1) Assume the site-local address is SL and the prefixes in the
         Site Prefix List are SP1, SP2, ... SPm.  The Site PLength of
         each of the prefixes is Length(SPj).

      2) First perform a regular reverse lookup of the IPv6 address.  If
         the lookup succeeds return success to the application.  If the
         lookup fails and the IPv6 address is not a site-local address
         report the failure to the application.

      3) When the reverse lookup of a site-local address fails use the
         Site Prefix List to construct global addresses corresponding to
         the site-local address.  This is done by taking each entry in
         the Site Prefix List and using it to construct a global
         address.  For each of the SPj concatenate the first Length(SPj)
         bits from SPj and the last (128 - Length(SPj)) bits from SL to



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         form a new address.  Look up each of the resulting addresses
         until a match is found.

   For example, if the site-local address is:
           fec0::987:X:Y:W:Z1

   and the prefixes in the Site Prefix List are:
           2837:a:b::0/48
           2000:1:2::0/48

   The addresses that should be tried in the reverse lookup are:
           fec0::987:X:Y:W:Z1
           2837:a:b:987:X:Y:W:Z1
           2000:1:2:987:X:Y:W:Z1



9.  MULTI-SITED NODES

   A node potentially connected to multiple sites needs to be able to

    o Detect that is it multi-sited.

    o Be configured with the appropriate AAAA/A6 records in the DNS.

    o Be able to distinguish between the different sites when
      originating applications.


   An alternative to multi-sited nodes is to not use any site-local
   addresses for the node "close" to the site boundary (i.e. and not
   list any site-local addresses in the DNS for that node).  This will
   force all traffic to and from that node to use global addresses
   (except those few cases where link-local addresses are used).



9.1.  Detecting that a Node is Multi-sited

   A possible algorithm for detecting when a node is multi-sited is as
   follows:

      1) Inspect the Site Prefix List for all interfaces.

      2) If an interface has no Site Prefix List entry ignore that
         interface.

      3) If two or more interfaces have one or more common Site Prefix



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         List entries group those interfaces together.

      4) If the result is more than one group of interfaces the node is
         considered to be multi-sited.

   If the node detects that it is multi-sited and does not contain
   support for site-local addresses in this environment it must at a
   minimum log an event.  It may also attempt to remove any site-local
   addresses assigned to it from the DNS to avoid communication failure
   should other nodes attempt to communicate with it using site-local
   addresses.



9.2.  Address Records for Multi-sited Nodes

   A given AAAA/A6 RRset can only contain site-local addresses for one
   site, since the site is implicit in the association between the
   global and site-local addresses contained in the same RRset.

   This implies that a multi-sited node that have a single domain name
   with AAAA/A6 records for interfaces in multiple sites can not have
   site-local addresses in that RRset.

   The multi-sited node can have a different domain name for each site
   to which it is connected, in order to enter site-local AAAA/A6
   records in the DNS.

   For example, a multi-sited node connected to two sites:
   Site1:
           Address 2000:1:2:987:X:Y:W:Z1   (A1)
           Address fec0::987:X:Y:W:Z1      (A2)
           Address 2000:1:2:34:X:Y:W:Z2    (A3)
           Address fec0::34:X:Y:W:Z2       (A4)
           Site prefix 2000:1:2::0/48
   Site2:
           Address 4444:a:b:34:X1:Y1:W1:Z3 (A5)
           Address fec0:::34:X1:Y1:W1:Z3   (A6)
           Site prefix 4444:a:b::0/48
   This node could have 3 different host names:
      foo.bar.site1.tla which list the AAAA/A6 records A1 through A4

      foo.site2.tla which list the AAAA/A6 records A5 through A6

      foo.net which list the global AAAA/A6 records A1, A3 and A5.






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9.3.  Distinguishing Between Different Sites

   A multi-sited node needs to take additional care in applications and
   in the protocol stack to qualify any site-local addresses with the
   site unless all applications always associate an interface with each
   IP address.  (For instance, the use of getaddrinfo() as specified in
   [BSD-API] allows the transparent passing of a site-id to the TCP/IP
   stack in the sin6_scope_id without modifying the applications.)  It
   is recommended (but not required) that implementations which operate
   at site boundaries have such support.

   If the implementation does not have such support it must not pass any
   site-local addresses to the applications since it will not be
   possible for the IP layer to determine which site (i.e., the set of
   interfaces attached to a site) to originate the packet on.



10.  SECURITY CONSIDERATIONS

   Router Advertisements are not required to be authenticated and even
   if they are authenticated it is unclear whether or not there would be
   a mechanisms to verify the authority of a particular node to send
   Router Advertisements.

   Neighbor Discovery uses the rule of HopCount 255 (set to 255 on
   transmit and verified to be 255 on reception) to drop any Neighbor
   Discovery packets that are sent non-neighboring nodes.  This limits
   any attack using ND to the neighbors.

   Without authentication and authorization this new mechanisms
   introduces a new type of denial of service attack.  A node on the
   link can send a router advertisement listing site prefixes that are
   in fact not part of the site.  For instance, it could advertise some
   other sites prefix as a site prefix.  Such an attack would result in
   all nodes on the link to fail initiate any new communication with any
   node in that site since they would accept the site-local AAAA/A6
   records.

   Also there is the possibility to return incorrect information for the
   reverse lookup of IPv6 addresses.  A node on the link can send a
   router advertisement listing site prefixes that are in fact not part
   of the site.  For instance, it could advertise an incorrect site
   prefix (e.g. a:b::0/48) which would make the reverse lookup of the
   site local address fec0::X lookup a:b::X.

   This could be viewed as allowing some form of indirect spoofing of
   the addresses returned by the DNS independent whether or not the DNS



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   itself is secure.  Thus introducing a secure DNS [DNSsec] would not
   remove this form of "address spoofing".  However, it seems like this
   threat is no worse than the other threats in [DISCOVERY] where any
   node on the link can intercept all packets sent on the link.

   The packets used to discover site prefixes, just like all other
   Neighbor Discovery protocol packet exchanges, can be authenticated
   using the IP Authentication Header [IPv6-AUTH].  A node SHOULD
   include an Authentication Header when sending Neighbor Discovery
   packets if a security association for use with the IP Authentication
   Header exists for the destination address.  The security associations
   may have been created through manual configuration or through the
   operation of some key management protocol.

   Received Authentication Headers in these packets, just like all
   Neighbor Discovery packets, MUST be verified for correctness and
   packets with incorrect authentication MUST be ignored.

   Confidentiality issues are addressed by the IP Security Architecture
   and the IP Encapsulating Security Payload documents [IPv6-SA, IPv6-
   ESP].






REFERENCES


     [KEYWORDS] S. Bradner, "Key words for use in RFCs to Indicate
             Requirement Levels", RFC 2119, March 1997.

     [IPv6] S. Deering, R. Hinden, Editors, "Internet Protocol, Version
             6 (IPv6) Specification", RFC 2460, December 1998.

     [ADDR-ARCH] S. Deering, R. Hinden, Editors, "IP Version 6
             Addressing Architecture", RFC 2373, July 1998.

     [SCOPE-ARCH] S. Deering, B.Haberman, B.Zill, "IP Version 6 Scoped
             Address Architecture", draft-ietf-ipngwg-scoping-arch-
             01.txt, July 2000.

     [DISCOVERY] T. Narten, E. Nordmark, and W. Simpson, "Neighbor
             Discovery for IP Version 6 (IPv6)", RFC 2461, December
             1998.

     [ADDR-TODAY] B. Carpenter, J. Crowcroft, Y. Rekhter, "IPv4 Address



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             Behavior Today", RFC 2101, February 1997.

     [GSE-EVAL] M. Crawford, A. Mankin, T. Narten, J. Stewart, L. Zhang,
             "Separating Identifiers and Locators in Addresses:  An
             Analysis of the GSE Proposal for IPv6", Internet Draft,
             draft-ietf-ipngwg-esd-analysis-06.txt, July 2000.

     [ROUTER-RENUM] M. Crawford, and R. Hinden, "Router Renumbering for
             IPv6", RFC 2894, August 2000.

     [ADDRCONF] S. Thomson, T. Narten, "IPv6 Address Autoconfiguration",
             RFC 2462, December 1998.

     [IPv6-SA] R. Atkinson.  "Security Architecture for the Internet
             Protocol".  RFC 2401, November 1998.

     [IPv6-AUTH] R. Atkinson.  "IP Authentication Header", RFC 2402,
             November 1998.

     [IPv6-ESP] R. Atkinson.  "IP Encapsulating Security Payload (ESP)",
             RFC 2406, November 1998.

     [DNSsec] D. Eastlake, C. Kaufman, "Domain Name System Security
             Extensions", RFC 2535, March 1999.

     [MOBILE-IPv6] D.B. Johnson, C. Perkins, "Mobility Support in IPv6",
             Internet Draft, draft-ietf-mobileip-ipv6-07.txt, March
             1999.

     [BSD-API] R. Gilligan, S. Thomson, J. Bound, W. Stevens, "Basic
             Socket Interface Extensions for IPv6", RFC 2553, March
             19999.



AUTHOR'S ADDRESS

        Erik Nordmark
        Sun Microsystems Laboratories
        29, Chemin du Viuex Chene
        38240 Meylan, France

        phone: +33 (0)4 76 18 88 03
        fax:   +33 (0)4 76 18 88 88
        email: nordmark@sun.com






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APPENDIX A: CHANGES SINCE PREVIOUS DRAFT


   The following changes have been made since version 04 of the draft.

    o Changed author's address.


   The following changes have been made since version 03 of the draft.

    o Removed remaining use of "create site-local" to make it clear that
      site-local addresses are stored in the DNS and "filtered" by the
      nodes.


   The following changes have been made since version 02 of the draft.

    o Moved the Site PLength field in the option format to make it
      easier for [ROUTER-RENUM] to include the field in its option
      format.
    o Changed the rules about suppressing global addresses to only
      suppress the ones that match the site prefixes.
    o Refined the rules and text to handle case when site not connected
      to the Internet i.e. when the DNS returns only site-local
      addresses.
    o Specified that a multi-sited node that have a single domain name
      can use site-local addresses on at most one site.  This is to
      ensure that one AAAA/A6 RRset contains site-local addresses for at
      most one site.  The multihomed node must have a different domain
      name for each site in it wants to use site-local addresses in all
      its sites.
    o Defined "multi-sited node"
    o Clarified that a multi-sited node can, instead of ignoring all
      site-locals, pass the full AAAA/A6 RRset (include the site-local
      addresses) for nodes in directly attached sites *IF* the
      applications and protocol stack can ensure that the communication
      will use the proper site. (For instance, using mechanisms like
      getaddrinfo() and the sin6_scope_id to pass the local site
      identifier to the protocol stack transparent to the application.)
    o Changed references from AAAA to AAAA/A6.


   The following changes have been made since version 01 of the draft.

    o Stated the assumptions on what a "site" is and how it is
      configured.

    o Changed the document to store site-local addresses in the DNS and



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      use filtering do ignore site-local addresses unless the sender and
      receiver can be determined to belong to the same site.

    o Added text describing interaction with mobile IP.

    o Added rules for ignoring site-local entries from the DNS

    o Make "turn off at site boundary" implementation dependent.

    o Changed 'S' bit in prefix option not to conflict with [MOBILE-
      IPv6].


   The following changes have been made since version 00 of the draft.

    o Removed mention of routing protocols.

    o Made the formed site-local addresses replace the global addresses
      in the list returned to the application.  This change prevents the
      "accidental" use of a global address when the application tries
      all of the returned addresses and for whatever reason it could not
      reach the node when it tried the site-local address(es).

    o Added text describing how to the mechanism is automatically
      disabled on nodes which are  Multihomed to multiple sites.

    o Updated list of open issues.
























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